react-spring/packages/core/src/interpolate.ts

import { FluidValue, deprecateInterpolate } from '@react-spring/shared'
import {
  Constrain,
  OneOrMore,
  Animatable,
  ExtrapolateType,
  InterpolatorConfig,
  InterpolatorFn,
} from '@react-spring/types'
import { Interpolation } from './Interpolation'

/** Map the value of one or more dependencies */
export const to: Interpolator = (source: any, ...args: [any]) =>
  new Interpolation(source, args)

/** @deprecated Use the `to` export instead */
export const interpolate: Interpolator = (source: any, ...args: [any]) => (
  deprecateInterpolate(), new Interpolation(source, args)
)

/** Extract the raw value types that are being interpolated */
export type Interpolated<T extends ReadonlyArray<any>> = {
  [P in keyof T]: T[P] extends infer Element
    ? Element extends FluidValue<infer U>
      ? U
      : Element
    : never
}

/**
 * This interpolates one or more `FluidValue` objects.
 * The exported `interpolate` function uses this type.
 */
export interface Interpolator {
  // Tuple of parent values
  <Input extends ReadonlyArray<any>, Output>(
    parents: Input,
    interpolator: (...args: Interpolated<Input>) => Output
  ): Interpolation<Output>

  // Single parent value
  <Input, Output>(
    parent: FluidValue<Input> | Input,
    interpolator: InterpolatorFn<Input, Output>
  ): Interpolation<Output>

  // Interpolation config
  <Out>(
    parents: OneOrMore<FluidValue>,
    config: InterpolatorConfig<Out>
  ): Interpolation<Animatable<Out>>

  // Range shortcuts
  <Out>(
    parents: OneOrMore<FluidValue<number>> | FluidValue<number[]>,
    range: readonly number[],
    output: readonly Constrain<Out, Animatable>[],
    extrapolate?: ExtrapolateType
  ): Interpolation<Animatable<Out>>
}